1. Field of the Invention
This invention relates generally to polyester foam materials and, more specifically, to resilient polyester foams.
2. Description of the Prior Art
Polyesters may be broadly defined as macromolecular compounds having a plurality of carboxylate ester groups in their skeletal structures. Polyesters, as so defined for use herein, are to be distinguished from other ester-containing polymers (e.g. cellulose esters, polyvinyl esters, and polyacrylates) wherein carboxylate groups are present in substituent entities pendant from the backbone of the polymer. Polyesters have been known to science for many years and have been used in recent years in such diverse applications as coatings, films, fibers, molding and casting compounds, and as intermediates in chemical reactions. Polyester foams based on unsaturated acids and unsaturated monomeric cross-linking materials have also been known and used. Foams made from polyesters whose polymeric structure is not dependent for development on unsaturation within its substituent monomers, however, have received scant if any attention in the past. And the same is specifically true of resilient foams made from such polyesters.
Most polyesters finding use today are linear polymers as opposed to three dimensional polymers. Three dimensional polymers are, of course, polymers having cross-links between the essentially linear polymeric structures forming the skeletal backbones of the molecules. There can be some branching within the skeletal backbones without having a departure from essential linearity.
Polyesters have been synthesized in the past from a variety of reactants through the use of several reaction schemes. The most direct synthesis is the esterification of a dicarboxylic acid with a glycol. (Dicarboxylic acids and glycols are sometimes referred to, respectively, as dibasic acids and dihydroxy compounds). In this scheme, the dicarboxylic acid/glycol mixture is heated until condensation occurs. The products of the condensation reaction are polyester and water.
Polyesters can also be prepared by ester exchange reactions.
A third general method of preparing polyesters, and the one which is most useful in the practice of the instant invention, is the polycondensation of polyols with acyl halides such as diacid chlorides. The products of this polycondensation reaction are, of course, the polyester and a hydrogen halide such as hydrogen chloride. Depending upon the physical properties of the reactants, the acyl halide-polyol reaction is frequently conducted in the presence of an inert solvent such as chlorobenzene or a chlorinated biphenyl. A stream of an inert gas is frequently passed through the reaction system to remove the gaseous hydrogen halide. It is also possible to conduct the acyl halide polyol reaction without the use of solvents if the reactants are low melting compounds which can form a homogeneous mixture. In either case, it is essential that all components of the reaction system be free of moisture since water hydrolizes the acyl halide thereby terminating the polymerization reaction.